Generally, the so-called suspension device for motorcycles is constructed and arranged in such a manner that a rear wheel is rotatively supported by rear ends of two right and left side swing arms front ends of which are pivotally secured to a rear low-portion of the vehicle frame, and at each side of the rear wheel a spring member including a shock absorber is supported between a rear low-portion of the swing arm and a rear top-portion of the vehicle frame, whereby upward and downward movement of the rear wheel with respect to the vehicle frame and damping action against said movement are caused. In the suspension device as mentioned above, however, there are various defects such as that the spring members become considerably obstructive members because of sideward projection thereof at both sides of the rear wheel, underward weight imposed on each swing arm is considerably large because weight of each spring member should be supported by corresponding swing arm, and use of two spring members causes highly expensive design from viewpoint of maintaining balance of the vehicle.
The following patents have been proposed in order to eliminate the defects involved in the general suspension device as mentioned above.
British Pat. No. 104022
U.S. Pat. No. 3,907,332
The suspension device according to said prior patents have a construction such that center portion of a link lever or bell-crank extending frontward and rearward is pivotally supported by a rear top-portion of the vehicle frame, rear portions of two swing arms positioned respectively at each side of the rear wheel and fork-shaped rear ends of said link lever are pivotally connected respectively by two rod members positioned respectively at each side of the rear wheel, and a spring member is supported between front end of the link lever and rear low-portion of the vehicle frame. According to the structure as mentioned above, upward and downward movements of the swing arms are transmitted to the spring member itself through the rod members and the link lever, whereby the following merits are caused. That is to say, members located at both sides of the rear wheel are only the relatively thin and light rod members which do not obstruct said side portions of the rear wheel, use of only one spring member becomes possible thereby to cause low cost of the suspension device, it becomes possible to design the suspension device in such a manner that the force imposed on the rear wheel is transmitted onto the spring member in a suitable lever-ratio, and an effective ratio of the reaction force imposed on the rear wheel from the spring member can be gradually enlarged by utilizing variation of the lever-ratio, said variation being caused by deflection of the link lever, whereby the so-called rising rate spring system can be mechanically obtained.
In the case of the suspension device of the prior patents as mentioned above, however, a new disadvantageous problem occurs. That is to say, since only one spring member is used, the spring member becomes large and wire diameter and wound diameter of the spring member become thick and large. Said spring member have to be arranged within rear portion of the vehicle frame, but other members such as air cleaner should be arranged at said rear portion, and therefore considerably defective influence would be imposed on arrangement of various parts or on maintenance of the vehicle, because said rear portion is originally narrow.